1. Field of the Invention
The present invention relates to a microwave oven and a method of operating a microwave oven.
2. Description of the Related Art
A typical microwave oven includes a cavity for receiving a load to be heated. Microwave energy is generated by means of a magnetron, and the magnetron is operatively connected by a waveguide to feeding ports for feeding microwave energy into the cavity.
Various efforts have previously been made to determine the presence and/or nature of a load placed in the cavity. One reason for determining the presence and/or nature of a load in the cavity is that strong reflection from the cavity typically occurs when the cavity is empty. Such reflection may deteriorate or even destroy the magnetron. For example, WO 03/07929 discloses a method in which a microwave signal escaping from an orifice placed in the wall of the cavity is measured, and the measured signal is used for determining presence and/or nature of a load in the cavity. In case the measured signal corresponds to a level above a level considered safe for the magnetron, the oven is stopped.
Another purpose for determining presence and/or nature of a load in the cavity is to adapt or select the heating cycle in response to such determination. For example, U.S. Pat. No. 4,210,795 discloses a method for regulating the output power of a magnetron in a microwave oven, wherein the reflection coefficient within a feeding waveguide is measured and a control signal is generated to switch the magnetron between first and second levels of power output when the reflection coefficient reaches a predetermined value indicating that a food item in the oven has begun to defrost.
However, the predictability or resolution of these prior methods for determining the presence and/or nature of a load in the microwave cavity is low, thereby limiting the benefits of using these methods in practice, for example in domestic microwave ovens.